The present invention relates to a data transmission system of a key telephone system.
In a conventional key telephone system, a main unit of the key telephone system is connected to a key telephone set through a pair of speech lines and a pair of signal lines.
When the pulse signal which is changed simply in the negative direction is used, transformer characteristics and distributed capacitance of the signal lines cause decreases in reference levels upon transmission of the pulse signals. When power is transmitted from the main unit to the key telephone set, a worse influence is effected to keep the above-mentioned reference level constant. At the receiving end, detection of the pulse signal by comparison with a reference voltage is inaccurate. In order to solve this problem, U.S. Pat. No. 4,564,726 (issued on Jan. 14, 1986) describes a means for transmitting data by using a positive- and negative-direction pulse signal, that is by using the double current method so as to cancel the changes in reference levels.
However, when the positive- and negative-direction pulse signals are used, the number of positive pulses is not always equal to that of negative pulses at the time of data coding due to the contents of data. If the number of positive pulses is larger than that of negative pulses and vice versa, the DC component is generated according to the average value of the pulses. Therefore, the reference level is changed, and the detection conditions at the receiving end become undesirably unstable.